Natural silicon is composed of three kinds of stable isotopes (hereinafter, referred to as 28Si, 29Si, 30Si, respectively) of which the mass numbers are 28, 29 and 30. The stable isotopes 28Si, 29Si, 30Si exist in a ratio of 92.23%, 4.67% and 3.10% (atom %), respectively.
There is reported a single crystal prepared by highly enriched a single silicon isotope having improved thermal conductivity as compared with one having a natural ratio. Therefore, the single crystal is reasonably expected as a material contributing to higher speed, miniaturization, lower power consumption and improved stability of LSI. Silicon prepared by completely removing 29Si having a nuclear spin is a material which is most likely to realize a quantum computer proposed to allow ultrahigh-speed calculation.
It is necessary to establish a technique for mass manufacturing isotope-separated silicon at a low cost for promoting research and development in such a field, developing a new application, and expanding sales of devices or the like using isotope-concentrated silicon.
Examples of conventional techniques for the silicon isotope enrichment method include a distillation method, a centrifugal separation method, a laser decomposition method, an isotope exchange reaction method, and the like. However, since the distillation method provides an extremely small separation factor and a very long tower required for separation, the distillation method is hard to be accomplished as a commercial process. The centrifugal separation method requires a large-sized centrifugal separators and therefore high cost. The laser decomposition method provides a low yield, and therefore is unsuitable for mass production.
As the isotope exchange reaction method, there is disclosed, for example, a method using a complex of halogenated silicon or the like and C1-C4 alkyl alcohol or the like in U.S. Pat. No. 6,146,601 (Patent Document 1) Since silicon tetrafluoride, which is a kind of the halogenated silicon, is generally known to be reacted with water to be hydrolyzed (see the following chemical formula), the method prevents the hydrolysis using a prescribed organic solvent as a donor compound.3SiF4+2H2O→2H2SiF6+SiO2  [Chemical Formula 1]
However, silicon tetrafluoride is known to be reacted with alcohols by the same mechanism as that of water under some operating conditions to be decomposed and generate alkyl orthosilicate and alkoxy fluorosilicate. Therefore, the chemical exchange method may not perform a stable isotope separation operation. Furthermore, the use of an organic solvent such as C1-C4 alkyl alcohol requires explosion-proof facilities and causes the increase of manufacturing cost.
Patent Document 1: U.S. Pat. No. 6,146,601